Many aero-engines adopt a dovetail style of fan blade root which locates in a corresponding slot formed in the rim of the fan disc. During service operation, the fan assembly is subject to a complex loading system, consisting of centripetal load, gas-bending and vibration. The dovetail geometry copes particularly well with this kind of loading conditions.
On assembly, the blades are “chocked” up to mate the flanks of the corresponding dovetail slots (in the absence of any centrifugal force when static) by inserting a slider beneath the blade root. When the rotor assembly is spinning, the blades are restrained radially by the dovetail slots, which are sized according to mechanical rules based on extreme load cases.
To prevent the blades moving axially forward or rearward a number of approaches can be employed. One is to use a solid block or plate of metal inserted into machined grooves in the disc either at the front and back of the dovetail slot or mid slot (which requires a corresponding groove machined into the blade root). This approach relies on the shear strength of the plates (and disc grooves) to withstand any axial force placed on them. The plates are sized on the worst case of either large bird impact or trailing blade impact following a fan blade off event.
The large forces seen during these extreme cases lead to a thick plate design and a correspondingly large extension of the disc. This requires larger and more expensive disc forging and increases the disc machining time. In addition, the extension: adds weight and therefore increases specific fuel consumption; can use up engine space and encroach on adjacent components; and can lead to pumping and windage, creating a secondary airflow and associated temperature increase. Further, the shear plate produces a larger part count, which increases costs and assembly time.
The mid slot approach requires machining of the blade root to accommodate the plate, which breaks through the dovetail flanks. This can be acceptable in the case of a metal blade, but may cause issues in a composite blade, where the groove in the blade root is typically perpendicular to the fibre plies in the root and has sharp edges, which may cause stress concentrations. Breaking the flanks can also require the blade root to be extended axially to meet acceptable crushing stress limits (which again lead to a corresponding increase in disc axial length).
Current blade retention approaches also offer little vibrational damping to the blade or disc.